1. Field of the Invention
The present invention relates to flame arresting or quenching plugs for use with transmitter housings.
2. Description of the Prior Art
Flammable mixtures many times are present in an atmosphere outside a pressure transmitter or in a process fluid which is being sensed by a pressure sensor in the transmitter. Various types of protection reduce the likelihood that an ignition of flammable gas inside the transmitter will ignite a flammable mixture outside the transmitter. Transmitter openings and covers for the openings are now made so as to cool ignited gas inside the transmitter housing as it is venting to a temperature below that needed to ignite a flammable mixture outside the transmitter. The ignited gasses in the transmitter are cooled as they travel along narrow passageways such as flanges or threads between a housing and a cover of the transmitter. As the hot gases move along the passageway, heat is transferred from the flame to the walls forming the passageway. If the passageway is narrow enough, more heat is transferred from the flame to the housing and cover than is produced by the flame, and the flame front cools. If the passageway is also long enough, the flame front can be cooled to a temperature low enough to quench the flame, thereby preventing ignited gases from moving further down the passageway and igniting a flammable mixture outside the transmitter.
Multiple protection can be provided in a single transmitter so that, even if one protection is damaged or fails, another protection arrangement still reduces the likelihood of igniting the flammable mixture outside the transmitter. It is desired in some applications that a flame quenching path effectively contain ignition of a flammable mixture inside the housing even after resilient seals such as O-rings and thin foil barriers such as isolator diaphragms have been damaged or removed, or after isolator liquids have leaked out, or welds have failed.
Flame quenching between the sensor, which includes electrical circuitry capable of generating a spark under fault conditions, and the outside of the transmitter is a problem because there is a need to expose the sensor to the process fluid pressure, and, in the case of a gauge pressure .transmitter, there is also a need to expose the sensor to atmospheric pressure outside the transmitter, as well. Flame quenching paths between sensors in a transmitter housing and the outside of the transmitter have been provided, but prior art arrangements such as class I threads are difficult to make in applications for holding a sensor in place and expensive to manufacture.
There is thus a need to provide an easily produced, low cost mounting for a sensor which exposes the sensor to the sensed pressure, while at the same time providing a simple, economical arrangement for containing gases ignited in the sensor cavity.